Effect of Continuous Mixer Design and Parameters on the Degradation of Polylactic Acid

Polylactic acid (PLA) has gained attention as a sustainable, compostable polyester, but process-induced degradation in single- and twin-screw extruders reduces PLA's molecular weight and affects its properties. In addition, PLA is often blended with other materials to improve its properties. A continuous mixer, which provides tighter control of shear levels and lower processing temperatures, produces less degradation of heat-sensitive polymers like polyvinyl chloride, but there is limited information about the effects of machine design and processing parameters. Therefore, this work investigated three parameters in the mixer section (rotor design, rotor speed, and orifice position) and screw speeds in the extruder section when processing PLA using a continuous mixer. The resultant PLA samples were characterized for their rheological, thermal, and chemical structure properties. It was found that higher rotor speeds and smaller orifice openings resulted in lower molecular weights, whereas varying the screw speed in the extruder did not significantly affect the molecular weight. Rotor design substantially impacted degradation, with rotors that provided lower shear stress and residence time producing very low reductions in molecular weight. Overall, this work provided insight on how to select rotors and processing parameters to reduce degradation of PLA for continuous mixer.